Cycles Material "Texture missing" in render?
Update!

Apparently it has something to do with an out of memory error (since this is just a background for  gigantic scene with hundreds of textures)

As you can see it doesn't render properly when all objects are visible


But as soon as I hide every object except for example a tree that isn't rendering properly, everything works as it's supposed to


Blender gives no CUDA errors though.. I'm really confused. 
I have 10GB or RAM installed and a GTX 550 ti with 1GB btw if that helps

SOLVED

Apparently there was indeed a CUDA out of memory error. It's fixed by just switching to CPU render.

Thanks everyone for their input!!!